1. Field of the Invention
The present invention relates to a seamless steel pipe excellent in hydrogen-induced cracking resistance (hereinafter referred to as “HIC resistance”), which is used as a line pipe having 5L-X70 grade or higher of American Petroleum Institute (API) Standard in strength level.
2. Related Art
In recent years, well conditions of an oil well for crude oil and a gas well for natural gas (hereinafter referred to as only “oil well and the like” generally) become severe and the transportation of the crude oil and natural gas has been performed under a severe environment. As the depth of water is increased, the well condition of the oil well and the like tends to contain CO2, H2S, Cl—, and the like in the ambient, and H2S is often contained in the crude oil and natural gas.
When the oil well and the like are in the seabed, as the depth of water is increased, an off shore pipe line is demanded for high strength and thick wall thickness to stand the water pressure on the seabed. As the off shore pipeline in such deep sea seamless steel pipes are usually used.
In a pipeline used for the transportation of crude oil or natural gas containing much H2S, not only corrosion of a surface of a steel material due to H2S, but also a fracture phenomenon of the steel material such as hydrogen-induced cracking or hydrogen-induced blistering or the like (hereinafter referred to as “HIC” generally) due to absorption of hydrogen generated by the corrosion into steel, are generated. This HIC is different from the sulfide stress corrosion cracking, which is conventionally recognized in a high strength steel, and does not depend on external stress so that the occurrence of HIC is recognized without external stress.
When such an HIC is occurred in a transporting pipeline, it may lead to a breakage accident of the pipeline. As a result a large scale environmental breakage due to leakage of crude oil or natural gas tends to occur. Accordingly, in the transporting pipelines for crude oil and natural gas, it is an important matter to prevent the occurrence of HIC.
The above-mentioned HIC is a steel material fracture phenomenon that inclusions such as MnS, Al2O3, CaO, CaS and the like existing in steel are changed, during the rolling of a steel material, to elongated ones in the rolling direction or crushed cluster-like ones, hydrogen absorbed into the interfaces between these inclusions and matrix steel is accumulated and gasified, cracks are generated by the gas pressure of the accumulated hydrogen, and these cracks propagate in steel.
To prevent the HIC, which exhibits such behaviors in steel, various steel materials for a line pipe has been proposed. For example, Japanese Patent Application Laid-open No. S50-97515 proposes steel for a line pipe in which Cu: 0.2-0.8% is added to steel having strength of X42-X80 grade in the API standard to form an anticorrosive film thereby preventing hydrogen from absorbing into the matrix steel.
Further, Japanese Patent Application Laid-open No. S53-106318 proposes a steel material for a line pipe in which Ca: excess 0.005%-0.020 or less %, which is comparatively a large amount, is added to steel and inclusion (MnS) in steel is spheroidized by a shape control by Ca treatment thereby reducing cracking sensitivity. Even at present HIC resistant steel has been produced based on these proposed technologies.
Further, since the principal use of the HIC resistant steel is a transporting pipeline for crude oil and natural gas, weldability is important. Thus a low-carbon steel is applied to the HIC resistant steel, but high strength steel is difficult to obtain due to the low C content of the steel. On the other hand, as mentioned above, consumers require for high strength materials. Thus, to satisfy the requirement, the following steps are often performed: after finish rolling a steel pipe by hot rolling, the steel pipe is heated and quenched, and subsequently tempered.
Such quenching and tempering treatment of a rolled steel pipe is effective for avoiding a ferrite and pearlite band-shaped microstructure in which HIC is liable to occur.
As mentioned above, in the steel material for a line pipe the weldability is important and high strength is required. Thus, after hot rolling, a rolled steel pipe is often subjected to be quenched and tempered. Further, in producing a seamless steel pipe, from the view points of a suppression of an increase in equipment costs and the production efficiency, it has been considered to adopt a treatment applying quenching and tempering after soaking, without cooling a finish-rolled steel pipe to Ar3 point, by directly connecting a pipe rolling line to a heat treatment line (hereinafter sometimes referred to as only “inline quenching/tempering (QT)”).
Accordingly, to improve the HIC resistance of a high strength steel material for a line pipe, a seamless steel pipe of a high strength material was produced by quenching and tempering after soaking without cooling the rolled steel pipe to Ar3 point after hot rolling by the use of a previously proposed steel in which inclusions (MnS) are shape-controlled by Ca treatment. However, the occurrence of HIC exhibiting a form of an intergranular fracture was observed. Thus, even if the HIC resistant steel proposed in the above-described Japanese Patent Application Laid-open No. S53-106318 and the like was applied to a high strength steel, the HIC resistance is not necessarily improved.